OTOH, I think punching a pillar is pure cheese and wouldn't allow it.
I'll blatantly admit it is pure cheese LOL
BUT it is the fault of the monk's design and completely within RAW. I even wrote it up as Hjalman venting his frustration in hitting the pillar first, informing Vecna of what is coming. It is sort of showboating and psyching himself up for the flurry of blows, which oddly enough even landed a critical hit! He
knows what he can do with flurry and it will stop Vecna, who cheesed him once before by teleporting away.
Now, what about this: what if Vecna had a minion next to him, and Hjalman hit the minion first,
then used flurry of blows on Vecna. Would it have been cheese then? It is the same tactic, just lacking the minion...
As for the stealth...
In terms of the "RAW" when it comes to hiding, no amount of high stealth keeps you hidden if the conditions of stealth are not met (aka total cover or heavy concealment).
Every time Malek has made a stealth check he HAS had total cover, no LoS between him and Vecna. After trying to hide, he then moves out
just enough (5 feet) to get Vecna in his sight and shoots him.
Now, we don't know what Malek's attack roll was the first round because we weren't told the numbers, but I know for his second sneak attack he hit AC 20, which hits Vecna (barely) even counting the half-cover I gave Vecna for still having the pillar partially in Malek's way. In the third round Malek hit AC 28, easily hitting this time.
So, yes, as soon as Malek stepped out enough to shoot, Vecna
might have seen him, but since the stealth rolls beat his passive perception the attack had advantage either way. Again, JC has already ruled that when hidden you can move up to 5 feet, revealing yourself, and still have advantage if you attack
immediately after moving. That's what I've been going with and it seems like
@MonsterEnvy did as well.